Optical instruments for highly sensitive gas sensing are being developed for use in future environmental, industrial, and health monitoring applications. To measure multiple sensor channels in an optical spectroscopic measurement system, the conventional methods either use multiple data acquisition channels, or use a time-multiplexing method to connect multiple sensor output to the same data acquisition channels. These solutions result in a higher total system cost due to an increased number of data acquisition channels or less efficient use of the output signal from a sensor due to dead measurement time.
FIG. 1 is an illustration of an existing optical spectroscopic system for multiple gas components detection as disclosed by the publication “Compact Portable QEPAS Multi-gas Sensor”, Proc. of SPIE Vol. 7945, 2011. When the laser is modulated by a sinusoidal waveform at a frequency of f, the material absorption signals in the sensor output are at the harmonic frequency of 2f, and the signal intensity at 2f is proportional to a material concentration value in the sample. The processed data is transferred to other devices (i.e., to another computer via the RS232 communication port) for display and storage. The system also provides a monitoring port of the 2f signal to the user.
As shown in FIG. 1, each sensor (a modified quartz tuning fork called spectrophone which is a piezoelectric element) is represented by SPh1 to SPh4. A trans-impedance amplifier (TA) is attached to each sensor to convert the tiny piezoelectric current signal generated by the sensor into a voltage signal to be digitized. The voltage signals from the four sensors are combined by a device (COMM) into one 2f signal. This signal combining device can be a voltage combiner or a signal switch.
FIG. 2 shows that a lock-in amplifier is employed to measure the sensor signal intensity at the specified modulated frequency as disclosed by the publication: “Quartz-enhanced photoacoustic spectroscopy”, Opt. Lett. V27, 1902, 2002. Because the lock-in amplifier is well-known for its capability of extracting a signal with a known carrier frequency from an extremely noisy environment, depending on the reference frequency supplied to the lock-in amplifier, the system is capable of detecting one modulation frequency for one sensor at a time. However, the total measurement time of the system is the sum of the measurement time of each sensor channel.
In optical spectroscopic measurement, when the light energy is absorbed by the targeted sample material, some portion of the absorbed energy is converted into other forms of energy such as light of different wavelengths, ultrasonic waves and heat energy radiated from the sample. A special sensor can be used to detect this radiated energy from the sample. In order for the sensor to effectively differentiate the energy radiated by the sample from other environmental noise, the excitation light is often intensity modulated or wavelength modulated in time, and the radiated energy from the sample has the same modulation frequency. Because the sensor is often designed to have one fundamental resonant frequency, it is important to choose the intensity modulation frequency of the light source to match the resonant frequency of the sensor to produce the maximum detection efficiency.
As shown in FIG. 1, a sinusoidal waveform is frequently used to modulate the intensity or the wavelength of the light source used in optical spectroscopic measurement. Because the energy conversion process in the sample is usually not a perfect linear process, the sample radiated energy will contain the fundamental modulation frequency and harmonic components. A sensor with only one resonant frequency can only response to one frequency excitation and has much weaker or no response at the harmonic frequencies. The sample radiated energy at the harmonics of the modulated frequency is not detectable by the sensor and become wasted.
As discussed above, the existing systems are very complex and inefficient. Therefore, there is a need to solve the challenges in highly sensitive material analysis using optical spectroscopic method which require increased sample measurement channels, reduced system complexity and improved measurement efficiency and detection sensitivity.